In recent years, encoders obtaining position or speed information as electrical signals have become higher in resolution and have become able to handle higher speed rotations. The method of outputting signals from encoders is also changing from the conventional A/B phase output method to the method of transferring data by serial communication not governed by the resolution or rotational speed. Data transferred by serial communication includes not only position information, but also alarm information indicating an abnormality detected in the encoder. An abnormality of the encoder itself can be analyzed by that alarm information (see Japanese Patent Publication (A) No. 2005-233720).
When detecting an abnormality by an abnormality detection circuit, an encoder notifies the control apparatus of the abnormality as alarm information. Due to this, it is possible to identify the abnormal location in the encoder. However, if the communication circuit of the encoder breaks down, the communication cable breaks, the communication circuit of the control apparatus breaks down, etc., the control apparatus detects the communication abnormality and displays an alarm, but the communication is cut, so time is required for identification of the fault location. Further, if the fault location is mistakenly identified, the abnormality will reoccur even after the parts are replaced and further time will be taken for restoration of the apparatus.
In the past, when communication was cut, another encoder was connected to the communication cable and if communication was normally performed was examined to judge if the original encoder was abnormal. However, with this method, since the other encoder was in the initial state, an initial state alarm showing that the absolute position was not established was sent to the control apparatus. When the control apparatus recognized that the encoder was in the initial state, it ended up invalidating the origin information stored for displaying the absolute position of the original encoder. Therefore, at the time of restoration after identifying the abnormal location, an operation for resetting the origin was required.
When connecting another encoder to check the communication, sometimes the original encoder is not the cause of the communication abnormality. For example, sometimes a break in the communication cable is the cause. When the original encoder is not the cause of the communication abnormality, a new communication cable is installed, then the original encoder is attached to the servo motor. However, if reconnecting the original encoder to the control apparatus, since the origin information is invalidated, the origin has to be reset.
This operation for setting the origin requires the reproduction of the predetermined origin position of the table or machine, so depending on the configuration of the machine, complicated, precision work using reference jigs or dial gauges or other measuring devices becomes necessary.
Further, the other replacement encoder for finding the cause of an abnormality would be expensive. Use for finding the cause of an abnormality would be uneconomical. Further, if connecting another encoder, the machine might mistakenly be placed in the operating state, so the work would be accompanied with danger.